£50k reward offered for finding Parklife punch killer. 

Robert Hart, 26, died after Parklife Festival assault. 

Attacker remains unidentified despite £20k reward. 

Renewed appeal for info on BBC's Crimewatch Live. 

Victim's mom: "Robert loved music, should be alive." 

Incident sparked over inflatable doll at festival. 

Police seek man and woman involved in assault. 

Suspect described as mixed race, mid-20s, 6ft tall. 

£50k reward for information leading to conviction. 

Detectives urge festival attendees to share images. 

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